Investigating settlement responses in the transitory period between planktonic and benthic stages of invertebrates helps shape our understanding of larval dispersal and supply, as well as early adult survival. Turbulence is a physical cue that has been shown to induce sinking and potentially settlement responses in mollusc larvae. In this study, we determined the effect of turbulence on vertical swimming velocity and diving responses in competent eastern oyster larvae Crassostrea virginica. We quantified the behavioural responses of larvae in a moving flow field by measuring and analyzing larval velocities in a relative framework (where local flow is subtracted away, isolating the behavioural component) in contrast to the more common absolute framework (in which behaviour and advection by the flow are conflated). We achieved this separation by simultaneously and separately tracking individuals and measuring the flow field around them using particle image velocimetry in a grid-stirred turbulence tank. Contrary to our expectations, larvae swam upward even in highly turbulent flow, and the dive response became less frequent. These observations suggest that oyster larvae are stronger swimmers than previously expected and provide evidence that turbulence alone may not always be a sufficient cue for settlement out of the water column. Furthermore, at a population level, absolute velocity distributions differed significantly from isolated larval swimming velocities, a result that held over increasing turbulence levels. The absolute velocity distributions indicated a strong downward swimming or sinking response at high turbulence levels, but this observation was in fact due to downwelling mean flows in the tank within the imaging area. Our results suggest that reliable characterization of larval behaviour in turbulent conditions requires the subtraction of local flow at an individual level, imposing the technical constraint of simultaneous flow and behavioural observations.

Observations of waves, flows, and water levels collected for a month in and near a long, narrow, shallow ($ 3000 m long, 1000 m wide, and 5 m deep), well-mixed ocean inlet are used to evaluate the subtidal (periods > 30 h) along-inlet momentum balance. Maximum tidal flows in the inlet were about 1.5 m/s and offshore significant wave heights ranged from about 0.5 to 2.5 m. The dominant terms in the local (across the km-wide ebb shoal) along-inlet momentum balance are the along-inlet pressure gradient, the bottom stress, and the wave radiation-stress gradient. Estimated nonlinear advective acceleration terms roughly balance in the channel. Onshore radiation-stress gradients owing to breaking waves enhance the flood flows into the inlet, especially during storms.

No abstract

Observations of water levels, waves, currents, and bathymetry collected for a month at an unstratified tidal inlet with a shallow (1 to 2 m deep) ebb shoal are used to evaluate the asymmetry in flows and dynamics owing to inertia and waves. Along-channel currents ranged from À1.5 to 0.6 m/s (positive inland) inside the main (3 to 5 m deep) channel crossing the ebb shoal. Net discharge is negligible, and ebb dominance of the channel flows is owing to inflow and outflow asymmetries near the inlet mouth. Offshore wave heights ranged from 0.5 to 2.5 m. During moderate to large wave events (offshore significant wave heights >1.2 m), wave forcing enhanced onshore mass flux near the shoal edge and inside the inlet, leading to reduced ebb flow dominance. Momentum balances estimated with the water depths, currents, and waves simulated with a quasi 3-D numerical model reproduce the momentum balances estimated from the observations reasonably well. Both observations and simulations suggest that ebb-dominant bottom stresses are balanced by the ebb-dominant pressure gradient and the tidally asymmetric inertia, which is a sink (source) of momentum on flood (ebb). Simulations with and without waves suggest that waves drive local and nonlocal changes in the water levels and flows. Specifically, breaking waves at the offshore edge of the ebb shoal induce setup and partially block the ebb jet (local effects), which leads to a more onshore-directed mass flux, changes to the advection across the ebb shoal, and increased water levels inside the inlet mouth (nonlocal effects).Plain Language Summary Inflow (flood) and outflow (ebb) from coastal river inlets drive the exchange of nutrients, pollutants, and biota between inland waters and the ocean. Measurements and computer models of flows, water levels, and waves at New River Inlet, NC, were used to understand how tides and waves affect flood and ebb flow patterns. New River Inlet has a shallow ebb shoal, a pile of sand that extends from the mouth to nearly 1 km offshore. Flood flows over the shoal are shown to funnel into the inlet radially from all sides of the inlet mouth, whereas ebb flows leave the inlet in a concentrated jet. Owing to this asymmetry in flood versus ebb, tidally averaged flows on the ebb shoal were seaward directed. Breaking waves at the offshore edge of the ebb shoal changed the flows and water levels across the entire ebb shoal. The water level is elevated and flows toward the inlet are enhanced where breaking wave momentum fluxes were large (local effects). The enhanced flow toward the inlet mouth elevated water levels inside the inlet where wave breaking was minimal (nonlocal effects).

The purpose of this Engineering With Nature technical note (EWN TN) is to review previous studies of mangroves as a nature-based adaptation alternative for coastal protection and flood hazard mitigation.